Combine the olive oil, dried oregano, garlic powder, salt, and pepper in a medium bowl. Add chicken thighs and coat evenly. Cover and marinate for at least 30 minutes at room temperature, or up to 2 hours in the refrigerator.

While chicken marinates, prepare the creamy feta tzatziki by mixing Greek yogurt, crumbled feta cheese, lemon juice, minced garlic, salt, and pepper until smooth. Refrigerate until ready to serve. Prep vegetables: halve cherry tomatoes, dice cucumber (remove seeds and drain on paper towels for 10 minutes), and thinly slice red onion (soak in cold water for 5 minutes, then drain).

Heat a skillet over medium-high heat. Add marinated chicken thighs and cook for 5 to 7 minutes per side until golden brown and internal temperature reaches 165°F. If browning too quickly, reduce heat to medium.

Transfer cooked chicken to a cutting board and let rest for 2 to 3 minutes. Slice into thin strips against the grain (perpendicular to muscle fibers) for maximum tenderness.

Warm pita breads in a dry skillet for 30 seconds per side or wrap in damp paper towels and microwave for 15 to 20 seconds. Pat drained cucumber dry with paper towels.

Spread generous amount of creamy feta tzatziki across each pita. Arrange sliced chicken down the center. Top with tomatoes, cucumber, red onion, and fresh parsley. Roll pita around fillings and serve immediately.